The present invention relates to an optical transmission system and method.
At present optical transmission systems for transmitting data, for example, telephone lines, generally operate with digital signals for reasons which are well known. For example, a digital transmission signal can provide an improved signal-to-noise ratio which means that repeaters may be spaced at a greater distance. For this reason and others, digital optical transmission systems are generally utilized.
If, however, analog optical transmission systems could be utilized then they have the advantage of being able to carry very many signals multiplexed together which allows for an increased signal carrying capacity.
In such an analog system, the multiplexed signals can be considered to produce an overall signal which is similar to "white noise", and it can be statistically predicted that at intervals the peak signal will be very great, which effectively is produced when a large number of the signal peaks of the multiplexed signals are aligned. In an optical transmission signal there are normally repeaters which receive input signals, amplify these input signals and produce an output signal, and the repeaters include an optical signal source which is normally a laser. The laser has a minimum threshold signal level below which distortions occur. Clearly the laser cannot be driven so as to produce a negative signal. The laser can normally also be driven so as to produce a maximum signal of any particular value desired. The laser is driven, however, to produce an analog signal which varies about a mean signal value, the means signal value being set so as to be a particular point above the minimum threshold signal level. If, however, one of the very large peak signals is received by the repeater, then although the laser can reproduce and amplify the positive peak, if the peak value is greater than the difference between the mean signal level and the minimum threshold signal level, the negative-going or lower amplitude part of the analog signal will be cut off and distortion will arise.